Superconducting composite wire and cable, processor for fabricating them

ABSTRACT

A superconducting structural body comprising 
     a superconducting ceramics and 
     a metal sheath surrounding the superconducting ceramics, 
     the metal sheath including 
     an Ag portion and 
     a non-Ag metal portion, the Ag portion existing from inner to outer faces of the metal sheath, and the superconducting ceramics portion existing in the structural body and the non-Ag metal portion used as a structural material for the metal sheath as the outermost indirectly contacting each other through the Ag material.

TECHNICAL FIELD

This invention relates to a superconducting composite wire whichcomprises a metal sheath filled with a compound having a perovskitestructure, the compound comprising a Y-including rare earth element(hereinafter indicated by a symbol R), an alkaline earth metal(hereinafter indicated by a symbol A), copper (Cu) and oxygen (O)(hereinafter, the compound being referred to as a superconductingceramics), to a superconducting composite cable which comprises aplurality of such superconducting wires bundled to each other, and to aprocess for fabricating them.

BACKGROUND ART

Generally, processes for fabricating wires using the superconductingceramics include the following steps:

(a) a step of providing starting powders, i.e., an R₂ O₃ powder, analkaline earth metal carbonate powder as the component A, and CuOpowder, each having an average grain size of not greater than 10 μm,compounding and mixing them in a predetermined compounding ratio, toobtain a mixed powder, calcining the mixed powder in the air or in anoxygenic atmosphere, at a temperature of from 850° to 950° C. to formsuperconducting ceramics having a perovskite structure, and grinding theceramics to obtain powder of an average grain size of not greater than10 μm,

(b) a step of filling a pipe of silver (Ag) with the superconductingpowder ground in the previous step, sealing the both ends of the pipeunder vacuum, and subjecting the silver pipe filled with the groundpowder to drawing operations, e.g., swaging, rolling with grooved rolls,processing with a die, or the like, to produce a wire having a diameterof not greater than 5 mm as shown in FIG. 1,

(c) a final step of sintering the superconducting ceramics powder filledin the wire and then subjecting the filled Ag pipe to heat-treatment inthe air or in an oxygenic atmosphere so that the ceramics can absorboxygen enough to be required, at a temperature of from 900° to 950° C.to produce a final product.

Subsequent to the above-described steps (a) and (b) have been performedthe following step:

(c') a step of bundling a plurality of the superconducting wires asshown in FIG. 1 and then covering the bundle with a tube made of Ag toform a cable, subjecting the cable to a processing with a die, ifrequired, and to a heat-treatment in the air or in an oxygenicatmosphere, at a temperature of from 900° to 950° C. for sintering thesuperconducting ceramics powder to produce a superconducting cable.

In the above described conventional step (c) or (c'), thesuperconducting ceramic powder is heat-treated to sinter it and toenable it to absorb oxygen. In this case, because the temperature of theheat-treatment is in the range of 900° to 950° C., which is close to themelting point of Ag, the strength of Ag decreases, and the Ag wiresfilled with the superconducting powders or the superconducting cablesbecome softened and tend to be easily bent or generate discontinuity orcut-off in the superconducting ceramics by careless bending. As aresult, the conventional superconducting wires and cables are difficultto handle, and they break sometimes down during heat-treatment.

Accordingly, one might consider the possibility of using, as coveringmaterials for the above described superconducting ceramics or wires,metals other than Ag (hereinafter, referred to as non-Ag metals), i.e.,materials having excellent strength at high temperatures, including, forexample, nickel alloys such as Inconel and Hastelloy, stainless steel,or the like. However, the non-Ag metals are disadvantageous because theycannot perform dispersion, penetration and discharge of oxygen.Specifically, in the superconducting wires and cables having sheath madeof non-Ag metals, bulges are formed in the wires or cables because ofoxygen released from the superconducting ceramics filled in the sheath.In the case of the wires, oxygen cannot be supplied to thesuperconducting ceramics when the superconducting ceramics filled in thewires are subjected to the final heat-treatment performed in the air orin an oxygenic atmosphere to sinter the superconducting ceramics. In thecase of the cables, when temperature of the cables is decreased afterthe sintering of the superconducting ceramics, the ceramics cannotabsorb oxygen.

Up to now, the metals other than Ag are practically unsuitable ascovering materials for the superconducting ceramics or the outermostcovering materials for the superconducting cables.

However, Ag has various problems in that it is very expensive, difficultto handle during high temperature heat-treatment, and has poor strengthat high temperatures. In particular, the strength of the cables at roomtemperature is unsatisfactory.

DISCLOSURE OF INVENTION

Accordingly, the present inventors have intensively investigated, and asa result they have found that the above-described problems can be solvedby adopting a structure in which a pipe or tube, which is used as theoutermost layer for forming the superconducting structural body, i.e.,the superconducting wires or cables, includes an Ag portion and a non-Agmetal portion, the Ag portion exists throughout from the inner face tothe outer face of the pipe or the tube, and the superconducting ceramicsportion disposed in the superconducting structural body and the non-Agmetal portion used as the material for the outermost layer indirectlycontact each other through the Ag materials.

More specifically, in the case of the superconducting wires, it has beenfound that when a composite pipe which is made of a material havingexcellent mechanical strength at high temperatures and which has the Agportion included from the inner to the outer faces of the wall thereof,with an inner layer of Ag formed at the inner face thereof, is filledwith the superconducting ceramics and then the obtained composite pipeis subjected to drawing, oxygen released from the superconductingceramics during the drawing can be diffused or discharged through the Agportion to the ambient, so that a bulge is hardly formed in the wire.Also, it has been found that even when the composite wire produced bydrawing is subjected to heat-treatment in the air or in an oxygenicatmosphere, the mechanical strength of the wire is retained by thenon-Ag metal portion and oxygen is supplied to the superconductingceramics via the Ag portion, resulting in that the deficiency of themechanical strength of the wire occurring during the heat-treatment canbe resolved.

In the case of the superconducting cable, it has been found that when acomposite tube is provided which includes an Ag portion and a non-Agportion, with the Ag portion existing from the inner to outer facesthereof, and a superconducting composite cable is produced by covering aplurality of the superconducting wires with the composite tube, theresulting composite tube has excellent mechanical strength at hightemperatures or room temperature, no bulge is formed in the compositecable because oxygen released from the superconducting ceramics isdiffused through the Ag portion and discharged to the ambient, andfurthermore after the superconducting ceramics filled in the cable issintered, oxygen can diffuse through the Ag portion of the compositetube into the superconducting ceramics and absorbed thereby.

This invention has been made on the basis of the above-describeddiscoveries.

Accordingly, it is a first object of this invention to provide asuperconducting structural body comprising a superconducting ceramicsand a metal sheath surrounding the ceramics, the metal sheath includingan Ag portion and a non-Ag metal portion, the Ag portion existing fromthe inner to outer faces of the sheath, and the superconducting ceramicsportion in the structure and the non-Ag portion used as a structuralmaterial for the metal sheath as the outermost layer being indirectlycontacted each other through the Ag portion.

It is a second object of this invention to provide a superconductingwire comprising a composite pipe having an inner layer made of Agmaterial and an outer layer including an Ag portion and a non-Agportion, the Ag portion of the outer layer being solid with the Agmaterial of the inner layer and exposed on the outer surface of thecomposite pipe, and a superconducting ceramics filled in the compositepipe.

It is a third object of this invention to provide a superconductingcomposite cable comprising a plurality of Ag-sheathed superconductingwires each being filled with superconducting ceramics, and a compositetube including an Ag portion and a non-Ag metal portion, the Ag portionexisting from the inner to outer faces thereof, the superconductingwires being covered with the composite tube.

It is a fourth object of this invention to provide a process forfabricating a superconducting composite wire comprising the steps of:

filling a composite tube with a superconducting ceramics, the compositetube including an inner layer made of Ag material and an outer layerincluding an Ag portion and a non-Ag portion, the Ag portion of theouter layer solid with the Ag material of the inner layer and exposed onthe outer surface of the composite tube,

sealing the both ends of the composite tube filled with thesuperconducting ceramics under vacuum,

drawing the sealed composite tube, and

heating the sealed composite tube drawn by the previous step.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a schematical perspective view, with portions broken awayfor clarity, of the conventional Ag-sheathed superconducting wire;

FIGS. 2 through 5 show schematical perspective views, with portionsbroken away for clarity, of superconducting composite wires according tothe present invention, respectively;

FIGS. 6 through 10 show schematical perspective views, with portionsbroken away for clarity, of superconducting composite cables accordingto the present invention, respectively, and

FIG. 11 shows a schematical perspective view, with portions broken awayfor clarity, of a composite pipe which is used as a superconductingcomposite wire of Example 1.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, not only Y-including rare earth elements basedsuperconducting ceramics oxide but also Bi-based superconductingceramics oxide such as BiSrCaCu₂ O_(y), Bi₂ Sr₂ Ca₂ Cu₃ O_(y),(Bi,Pb)SrCaCuO₁.5-2, Tl-based superconducting ceramics oxide such as Tl₂Ba₂ Ca₂ Cu₃ O_(x), Tl(Ba,Sr)₂ CaCuO, can be used as superconductingceramics, as far as the superconducting ceramics has a perovskitestructure and can easily release oxygen.

In the present invention, there can be used, as a material for the Agportion, not only pure Ag but also Ag-based alloys which contain one ormore metals such as platinum (Pt) that do not react with thesuperconducting ceramics and give a good influence upon the strength ofthe superconducting wires in amounts not preventing oxygen permeation.

Hereinafter, characteristics and advantages of this invention will beexplained in greater detail, with reference to the best modes of thisinvention illustrated in the attached drawings.

FIGS. 2 through 5 substantially illustrate the structure of thesuperconducting composite wire. In FIGS. 2 through 5, reference numeral1 denotes a portion of a metal other than Ag (non-Ag metal portion)having high mechanical strength at high temperatures. The non-Ag metalportion is preferably made of Austenite-based stainless steels,Ni-alloys such as Inconel, Hastelloy, or the like. In thesuperconducting composite wires according to the present invention, thereason why the inner layer is made of Ag materials is that when thesuperconducting ceramics is subjected to heat-treatment while contactingwith the non-Ag metal materials such as Austenite-based stainless steeland the like, a chemical reaction takes place to generate oxides such asNiO, FeO, Fe₂ O₃, Cr₂ O₃ or the like, resulting in the deterioration ofthe superconductive characteristics to a greater extent. Referencenumeral 2 denotes an Ag portion in which the inner layer and a part ofthe outer layer of the composite wire are made of Ag. The Ag portion 2in the outer layer is solid with the Ag of the inner layer 4, and isexposed on the outer surface of the composite wire. The superconductingcomposite wire is comprised by such composite pipe as described aboveand the superconducting ceramics 3 filled therein. As shown in FIGS. 2through 5, the Ag portions are exposed on a portion of the outer layerof the wire. Accordingly, when the Ag portions are subjected toheat-treatment for sintering the superconducting ceramics, oxygen issupplied by diffusion through the Ag portions in the outer layer to theinside of the wire. Thus, oxygen is supplied to the superconductingceramics.

The Ag portions in the outer layer may have an oval-shaped as shown inFIG. 2, a rectangular-shaped as shown in FIG. 3, a paralleledbelt-shaped as shown in FIG. 4, and a crack-shaped as shown in FIG. 5.However, the shape of the Ag portion 2 is not restricted to theabove-described ones, and may take any desired shape. Although each ofthe composite wires has a circular cross section, the cross section isnot restricted to the circular shape and may take any desired shape suchas a polygonal-shape including a square-shape, a rectangular-shape, ahexagonal-shape, an oval-shape, or the like.

FIGS. 6 through 10 each shows a schematical partial sectional view, withportions broken away for clarity, of the above described superconductingcomposite cables. In FIGS. 6 through 10, reference numeral 11 denotes anon-Ag metal portion, 12 denotes an Ag portion, 13 denotessuperconducting ceramics, and 14 denotes Ag. For comparison, aschematical partial sectional view, with portions broken away forclarity, of a superconducting wire including superconducting ceramics 13and Ag 14 shown in FIG. 1.

It is preferred that the above described materials for the non-Ag metalportions 11 be, as in the superconducting composite wire, Austenitebased stainless steels such as SUS304 or the like, Ni-alloys such asInconel, Hastelloy, and the like, respectively. The above-described Agportions 12 are present from the inner to the outer faces of thecomposite tube, and serve to diffuse oxygen released from thesuperconducting ceramics 13 to the ambient, and also supply oxygen tothe ceramics 13 from the ambient. Accordingly, because parts of thecomposite tube is made of the Ag portion 12, a bulge is not formed inthe composite tube since oxygen is released from the superconductingceramics 13 through the Ag portion 12. Also, oxygen from the ambient canbe passed through the Ag portion 12 in the composite tube to therebysupply oxygen to the superconducting ceramics 13.

As described above, the composite tube comprises the non-Ag metalportion 11 and the Ag portion 12. The Ag portion 12 may have anoval-shape as shown in FIG. 6, a rectangular-shape as shown in FIG. 7, acircular-shape as shown in FIG. 8, a paralleled belt-shape as shown inFIG. 9, and a spiral-shape as shown in FIG. 10, and the shape thereof isnot restricted to the above listed ones.

Furthermore, the superconducting composite cable as illustrated in FIGS.6 through 10 has a circular cross section. However, the shape of thecross section is not restricted to circular, and may be a polygonal suchas square, rectangular, hexagonal, or oval or any desired shape.

The above described metal sheath, concretely, the composite pipe in thesuperconducting composite wire, or the composite tube in thesuperconducting composite cable of this invention, can be produced byinserting a pipe of Ag or the like into a tube or a pipe each having aplurality of windows (perforations or the like) according to aconventional clad method or a method similar thereto and unifying themby drawing.

EXAMPLES

Next, the present invention will be concretely explained with referenceto the following examples.

EXAMPLE 1

As starting powder, Y₂ O₃ powder, BaCO₃ powder and CuO powder eachhaving an average grain size of 6 μm are provided and blended in a blendratio: Y₂ O₃ :15.13%, BaCO₃ :52.89%, CuO:31.98% (% by weight). Theblended powder are calcined in the air, at a temperature of 910° C., fora retention time of 10 hours, and then ground to obtain powder having anaverage grain size of 2.5 μm. Thus, a superconducting ceramics powder isproduced which has a composition of YBa₂ Cu₃ O₇ and has a perovskitestructure.

On the other hand, a composite pipe as illustrated in FIG. 11 isprovided. The composite pipe comprises an inner layer 4 made of Ag andan outer layer including an Ag portion 2 and Austenite based stainlesssteel 1 made of SUS304. The inner layer 4 has dimensions of thickness:0.5 mm×inner diameter: 5.0 mm, and the outer layer has dimensions ofthickness: 0.5 mm×outer diameter: 7.0 mm. The Ag portion 2 of the outerlayer has a ring-shaped and has diameter of 1.8 mm.

The above-described composite pipe is filled with the superconductingceramics powder 3, and then both ends thereof are sealed under vacuum.The sealed composite pipe is subjected to rotary-swaging to obtain awire having a diameter of 3.0 mm, and then the wire is subjected torolling with grooved rolls to obtain a superconducting composite wirehaving a diameter of 2.0 mm. The superconducting composite wire hasdimensions of thickness of inner layer thereof: 0.2 mm and thickness ofouter layer thereof: 0.2 mm. On the surface of the superconductingcomposite wire, the oval-shaped Ag portions are exposed.

The thus-obtained superconducting composite wire is subjected toheat-treatment in an oxygenic atmosphere, at temperature of 920° C., fora retention time of 15 hours. As a result, the superconducting compositewire does not cause the troubles encountered in the conventionalsuperconducting composite wire covered with Ag sheath. Specifically, thesuperconducting composite wire according to this invention is hardlybent during the heat-treatment, and is hardly broken down. Further, thewire is able to be easily handled.

The characteristics of the superconducting composite wire are measuredand the results of the critical temperature (Tc): 91° K. and criticalcurrent value (Jc): 3200 A/cm² are obtained.

For comparison, the superconducting wire covered with Ag sheath havingthe same dimensions as the superconducting composite wire according tothe present invention is provided, and the characteristics of thesuperconducting composite wire is measured and the results of thecritical temperature (Tc): 91° K. and critical current value (Jc): 3250A/cm² are obtained.

EXAMPLE 2

As starting powder, Y₂ O₃ powder, BaCO₃ powder and CuO powder eachhaving an average grain size of 6 μm are provided and blended in a blendratio: Y₂ O₃ :15.13%, BaCO₃ :52.89%, CuO:31.98% (% by weight). Theblended powder is calcined in the air, at a temperature of 910° C., fora retention time of 10 hours, and then ground to obtain powder having anaverage grain size of 2.5 μm. Thus, a superconducting ceramics powder isproduced which has a composition of YBa₂ Cu₃ O₇ and has a perovskitestructure.

A case of Ag having dimensions of inner diameter: 5 mm×thickness: 1mm×length: 200 mm is filled with the superconducting ceramics powderthus obtained, and both ends thereof are sealed under vacuum.Subsequently, the Ag case is subjected to cold rotary-swaging and coldrolling with grooved rolls. Finally, the Ag case is subjected to rollingwith grooved rolls to obtain forty superconducting wires each havingdimensions of diameter: 2.0 mm×length: 1700 mm.

On the other hand, a composite tube including Ag portions and SUS304Austenite based stainless steel portions and having dimensions of innerdiameter: 10 mm×thickness: 1.5 mm×length: 1000 mm is provided. Thecomposite tube is filled with twenty of the above describedsuperconducting wires, and then is subjected to processing with a die toobtain a superconducting composite cable having a diameter of 7 mm.Next, the superconducting composite cable is subjected to heat-treatmentin an oxygenic atmosphere, at temperature of 920° C., and for aretention time of 15 hours. The characteristics of the superconductingcomposite cable thus-obtained is measured and the results of thecritical temperature (Tc): 92° K. and critical current value (Jc): 4300A/cm² are obtained.

On the other hand, for comparison, a tube made of pure Ag havingdimensions of inner diameter: 10 mm×thickness: 1.5 mm×length: 1000 mm isfilled with the rest twenty superconducting wires, and then is subjectedto rolling with a die to obtain a superconducting cable having adiameter of 7 mm. The thus-obtained superconducting cable is subjectedto the same heat-treatment as that of the above-describedsuperconducting composite cable, and the characteristics of thesuperconducting cable are measured. The results of the criticaltemperature (Tc): 92° K. and critical current value (Jc): 4310 A/cm² areobtained.

INDUSTRIAL APPLICABILITY

Making a comparison between the superconducting composite wire and cableaccording to this invention and the conventional superconducting wireand cable, there is no difference therebetween as to the superconductivecharacteristics. However, in the superconducting composite wire andcable, the content of Ag, which is very expensive, can be reduced incomparison with the conventional wire and cable of this invention. Also,materials used have excellent strength at high temperatures.Accordingly, the superconducting composite wire and cable cause notrouble such as the breaking of the wire and cable, which wouldconventionally occur in the final heat-treatment step, so that they canbe produced with high productivity. Furthermore, because they haveexcellent strength at room temperature, they can be handled withoutspecial care, and can be serviced and examined at ease even afterinstallment.

We claim:
 1. A superconducting structural body comprisingasuperconducting ceramics and a metal sheath surrounding thesuperconducting ceramics, the metal sheath includingan Ag portion and anon-Ag metal portion, the Ag portion existing from inner to outer facesof the metal sheath, and the superconducting ceramics portion existingin the structural body and the non-Ag metal portion used as a structuralmaterial for the metal sheath as the outermost indirectly contactingeach other through the Ag material.
 2. A superconducting composite wirecomprisinga composite pipe having an inner layer made of Ag material andan outer layer includingan Ag portion and a non-Ag metal portion, the Agportion of the outer layer being solid with the Ag material of the innerlayer and exposed on the outer surface of the composite pipe, andsuperconducting ceramics filled in the composite pipe.
 3. Asuperconducting composite cable comprisinga plurality of superconductingwires covered with an Ag material and filled with superconductingceramics, and a composite tube includingan Ag portion, and a non-Agmetal portion, the Ag portion existing from inner to outer faces of thecomposite tube, the superconducting wires being covered with thecomposite tube.
 4. A process for fabricating superconducting compositewire comprising steps of:filling a composite tube with a superconductingceramics, the composite tube having an inner layer made of Ag materialand an outer layer includingan Ag portion and a non-Ag metal portion,the Ag portion of the outer layer being solid with the Ag material ofthe inner layer and exposed on the outer surface of the composite tube,sealing the both ends of the composite tube under vacuum, drawing thesealed composite tube, and subjecting the thus drawn tube drawn toheat-treatment.